Breast cancer is the most common female malignancy in most industrialized countries, as it is estimated to affect about 10% of the female population during their lifespan. Although its mortality has not increased along with its incidence, due to earlier diagnosis and improved treatment, it is still one of the predominant causes of death in middle-aged women. Despite earlier diagnosis of breast cancer, about 1-5% of women with newly diagnosed breast cancer have a distant metastasis at the time of the diagnosis. In addition, approximately 50% of the patients with local disease who are primarily diagnosed eventually relapse with the metastasis. Eighty-five percent of these recurrences take place within the first five years after the primary manifestation of the disease.
On presentation, most patients with metastatic breast cancer have only one or two organ systems involved. As the disease progresses over time, multiple sites usually become involved. Indeed, metastases can be found in nearly every organ of the body at autopsy. The most common sites of metastatic involvement observed are locoregional recurrences in the skin and soft tissues of the chest wall, as well as in axilla, and supraclavicular area. The most common site for distant metastasis is the bone (30-40% of distant metastasis), followed by lung and liver. Metastatic breast cancer is generally considered to be an incurable disease. However, the currently available treatment options often prolong the disease-free state and overall survival rate, as well as increase the quality of the life. The median survival from the manifestation of distant metastases is about three years.
Current methods of diagnosing and staging breast cancer include the tumor-node-metastasis (TNM) system that relies on tumor size, tumor presence in lymph nodes, and the presence of distant metastases as described in the American Joint Committee on Cancer: AJCC Cancer Staging Manual. Philadelphia, Pa.: Lippincott-Raven Publishers, 5th ed., 1997, pp 171-180, and in Harris, J R: “Staging of breast carcinoma” in Harris, J. R., Hellman, S., Henderson, I. C., Kinne D. W. (eds.): Breast Diseases. Philadelphia, Lippincott, 1991. These parameters are used to provide a prognosis and select an appropriate therapy. The morphologic appearance of the tumor can also be assessed but because tumors with similar histopathologic appearance can exhibit significant clinical variability, this approach has serious limitations. Finally assays for cell surface markers can be used to divide certain tumors types into subclasses. For example, one factor considered in the prognosis and treatment of breast cancer is the presence of the estrogen receptor (ER) as ER-positive breast cancers typically respond more readily to hormonal therapies such as tamoxifen than ER-negative tumors. Yet these analyses, though useful, are only partially predictive of the clinical behavior of breast tumors, and there is much phenotypic diversity present in breast cancers that current diagnostic tools fail to detect.
Traditional modes of cancer therapy include radiation therapy, chemotherapy, and hormonal therapy. Yet because of the difficulty in predicting the clinical course of early stage breast cancer from standard clinical and pathologic features, current practice is to offer systemic chemotherapy to most women even though the majority of these women can have a good outcome in the absence of chemotherapy. Chemotherapy has severe side effects and itself carries a 1% mortality rate, and thus unnecessary suffering and deaths could be avoided if patients could be divided into high and low risk subgroups. Thus, there exists a need for improved methods of classifying tumors for better prognosis and treatment selection.
Furthermore, although current therapies can often prolong the disease-free state and overall survival when used on high-risk patients, they are limited by their lack of specificity and the emergence of treatment-resistant cancer cells. Approximately two thirds of people diagnosed with cancer will die of their cancer within five years. Thus there is a great need for the identification of additional genes that can serve as selective therapies for the treatment of cancer.
Colorectal cancer is the third most common cancer and the fourth most frequent cause of cancer deaths worldwide. Approximately 5-10% of all colorectal cancers are hereditary with one of the main forms being familial adenomatous polyposis (FAP), an autosomal dominant disease in which about 80% of affected individuals contain a germline mutation in the adenomatous polyposis coli (APC) gene. Colorectal carcinoma has a tendency to invade locally by circumferential growth and for lymphatic, hematogenous, transperitoneal, and perineural spread. The most common site of extralymphatic involvement is the liver, with the lungs the most frequently affected extra-abdominal organ. Other sites of hematogenous spread include the bones, kidneys, adrenal glands, and brain.
The current staging system for colorectal cancer is based on the degree of tumor penetration through the bowel wall and the presence or absence of nodal involvement. This staging system is defined by three major Duke's classifications: Duke's A disease is confined to submucosa layers of colon or rectum; Duke's B disease has tumors that invade through muscularis propria and can penetrate the wall of the colon or rectum; and Duke's C disease includes any degree of bowel wall invasion with regional lymph node metastasis.
Surgical resection is highly effective for early stage colorectal cancers, providing cure rates of 95% in Duke's A and 75% in Duke's B patients. The presence of positive lymph node in Duke's C disease predicts a 60% likelihood of recurrence within five years. Treatment of Duke's C patients with a post surgical course of chemotherapy reduces the recurrence rate to 40%-50%, and is now the standard of care for these patients. Because of the relatively low rate of reoccurrence, the benefit of post surgical chemotherapy in Duke' B has been harder to detect and remains controversial. However, the Duke's B classification is imperfect as approximately 20-30% of these patients behave more like Duke's C and relapse within five years. Thus there is a clear need to identify better prognostic factors for selecting Duke's B patients that are likely to relapse and would benefit from therapy.
During normal animal development, cells of most or all tissues are derived from normal precursors, called stem cells (Morrison et al., Cell 88(3): 287-98 (1997); Morrison et al., Curr. Opin. Immunol. 9(2): 216-21 (1997); Morrison et al., Annu. Rev. Cell. Dev. Biol. 11: 35-71 (1995)). Stem cells are cells that: (1) have extensive proliferative capacity; 2) are capable of asymmetric cell division to generate one or more kinds of progeny with reduced proliferative or developmental potential; and (3) are capable of symmetric cell divisions for self-renewal or self-maintenance. In adult animals, some cells (including cells of the blood, gut, breast ductal system, and skin) are constantly replenished from a small population of stem cells in each tissue. The best-known example of adult cell renewal by the differentiation of stem cells is the hematopoietic system where developmentally immature precursors (hematopoietic stem and progenitor cells) respond to molecular signals to form the varied blood and lymphoid cell types.
Solid tumors are composed of heterogeneous cell populations. For example, breast cancers are a mixture of cancer cells and normal cells, including mesenchymal (stromal) cells, inflammatory cells, and endothelial cells. Classic models of cancer hold that phenotypically distinct cancer cell populations all have the capacity to proliferate and give rise to a new tumor. In the classical model, tumor cell heterogeneity results from environmental factors as well as ongoing mutations within cancer cells resulting in a diverse population of tumorigenic cells. This model rests on the idea that all populations of tumor cells would have some degree of tumorigenic potential. (Pandis et al., Genes, Chromosomes & Cancer 12:122-129 (1998); Kuukasjrvi et al., Cancer Res. 57: 1597-1604 (1997); Bonsing et al., Cancer 71: 382-391 (1993); Bonsing et al., Genes Chromosomes & Cancer 82: 173-183 (2000); Beerman H et al., Cytometry. 12(2): 147-54 (1991); Aubele M & Werner M, Analyt. Cell. Path. 19: 53 (1999); Shen L et al., Cancer Res. 60: 3884 (2000).).
An alternative model for the observed solid tumor cell heterogeneity is that solid tumors result from a “solid tumor stem cell” (or “cancer stem cell” from a solid tumor) that subsequently undergoes chaotic development through both symmetric and asymmetric rounds of cell divisions. In this stem cell model, solid tumors contain a distinct and limited (possibly even rare) subset of cells that share the properties of normal “stem cells”, in that they extensively proliferate and efficiently give rise both to additional solid tumor stem cells (self-renewal) and to the majority of tumor cells of a solid tumor that lack tumorigenic potential. Indeed, mutations within a long-lived stem cell population can initiate the formation of cancer stem cells that underlie the growth and maintenance of tumors and whose presence contributes to the failure of current therapeutic approaches.
Although great strides have been made understanding the genetic changes that lead to cancer (e.g. breast cancer and colorectal cancer), the lack of reliable tumor assay for de novo human cancer cells has hindered the ability to understand the effects of these mutations at the cellular level. Also, the lack of identified cancer markers for solid tumor stem cells has hindered the development of diagnostics and therapeutics for cancer patients (e.g. breast cancer patients). As such, what is needed is a reliable tumor assay as well as the identification of cancer markers for solid tumor stem cells.